Device for Continuous Operation of an Electrolysis Cell Having a Gaseous Substrate and Gas Diffusion Electrode

ABSTRACT

Various embodiments include a method for continuous operation of an electrolysis cell with a gaseous substrate, the method comprising: supplying an electrolyte to the electrolysis cell via an electrolyte feed; flowing the electrolyte out of the electrolysis cell into the gas space through a gas diffusion electrode; collecting the electrolyte from the electrolyte flow into the gas space in a collecting region in the gas space; and sucking the collected electrolyte out of said gas space via a connection between the gas space and electrolyte feed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2017/071292 filed Aug. 24, 2017, which designates the United States of America, and claims priority to DE Application No. 10 2016 217 989.6 filed Sep. 20, 2016, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to electrolysis. Various embodiments may include processes for continuous operation of an electrolysis cell with a gaseous substrate and/or devices for performing the process.

BACKGROUND

The combustion of fossil fuels currently covers about 80% of global energy demand. These combustion processes emitted about 34 032.7 million metric tons of carbon dioxide (CO₂) globally into the atmosphere in 2011. This release is the simplest way of disposing of large volumes of CO₂ as well (brown coal power plants exceeding 50 000 t per day). Discussion about the adverse effects of the greenhouse gas CO₂ on the climate has led to consideration of reutilization of CO₂. In thermodynamic terms, CO₂ is at a very low level and can therefore be reduced again to usable products only with difficulty.

In nature, CO₂ is converted to carbohydrates by photosynthesis. This process, which is divided up into many component steps over time and spatially at the molecular level, is copiable on the industrial scale only with great difficulty. The more efficient route at present compared to pure photocatalysis is the electrochemical reduction of the CO₂. A mixed form is light-assisted electrolysis or electrically assisted photocatalysis. The two terms can be used synonymously, according to the viewpoint of the observer. As in the case of photosynthesis, in this process, CO₂ is converted to a higher-energy product (such as CO, CH₄, C₂H₄, etc.) with supply of electrical energy (optionally in a photo-assisted manner) which is obtained from renewable energy sources such as wind or sun. The amount of energy required in this reduction corresponds ideally to the combustion energy of the fuel and should only come from renewable sources. However, overproduction of renewable energies is not continuously available, but at present only at periods of strong insolation and wind. However, this state of affairs will further intensify in the near future with the further rollout of sources of renewable energy.

The electrochemical reduction of CO₂ over solid-state electrodes in aqueous electrolyte solutions offers a multitude of product options that are shown in table 1 below, taken from Y. Hori, Electrochemical CO₂ reduction on metal electrodes, in: C. Vayenas, et al. (eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189.

TABLE 1 Faraday efficiencies for carbon dioxide over various metal electrodes Electrode CH₄ C₂H₄ C₂H₅OH C₃H₇OH CO HCOO⁻ H₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7

In operation of such electrolysis cells for CO₂ reduction, it has been found that electrolyte passes through a gas diffusion electrode (GDE) and leads to an electrolyte accumulation in the gas space. Both in flow-through and flow-by operation, the gas flow causes the electrolyte to lose its solvent, especially water, as a result of which depositions of salt occur in the gas space and, particularly disadvantageously, in the GDE itself. These lead to loss of selectivity and ultimately to the failure of the electrode or the electrolyzer.

DE 10 2012 204 041 A1, e.g. paragraphs [0007], [0008], [0041] and [0059], or DE 10 2013 011 298 A1, describes the mode of operation of an “oxygen-depolarized cathode”. Also described therein is the passage of electrolyte through the GDE. DE 10 2012 204 041 A1 additionally describes the possibility of blockage of the pores of the GDE.

The phenomenon of depositions of salt is particularly dominant here in electrolyzers that convert a gaseous substrate over a gas diffusion electrode back to gaseous substrates. There is thus a need for a process and an apparatus by which these problems caused by electrolyte passage can be reduced or remedied.

SUMMARY

The present disclosure describes a mode of operation in which salt migration takes place in such a way that an electrolysis cell nevertheless runs stably. More particularly, salting-out of electrolyte can be avoided in spite of passage through the electrode, and good electrolysis performance can be obtained over a long period.

For example, some embodiments may include a process for continuous operation of an electrolysis cell with a gaseous substrate, wherein an electrolyte is supplied to the electrolysis cell via an electrolyte feed, an electrolyte flow out of the electrolysis cell into the gas space takes place through a gas diffusion electrode, and the electrolyte from the electrolyte flow into the gas space is collected in a collecting region in the gas space, and the collected electrolyte is sucked out of said gas space, wherein the suction is effected via a connection between the gas space and electrolyte feed.

In some embodiments, the suction is effected in that the electrolyte feed exerts a suction effect on the gas space.

In some embodiments, the suction effect arises in that the connection between the gas space and the electrolyte feed comprises a Venturi nozzle through which the electrolyte is fed to the electrolysis cell.

In some embodiments, the electrolyte is sucked out of the gas space periodically.

In some embodiments, the periodic suction is effected via a closed-loop control mechanism that controls the periodic suction.

In some embodiments, the closed-loop control mechanism is mechanical.

In some embodiments, the closed-loop control mechanism comprises a float which is present in the collecting region in the gas space and enables outflow to the connection between the gas space and the electrolyte feed depending on the fill level of the electrolyte in the collecting region in the gas space, with periodic opening of the float.

In some embodiments, the suction is controlled by a valve that controls the connection between the gas space and the electrolyte feed.

In some embodiments, the valve is coupled to a fill level sensor for electrolyte in the gas space via a closed-loop controller, wherein the valve is controlled with reference to a measurement by a fill level sensor.

As another example, some embodiments include an apparatus for continuous operation of an electrolysis cell with a gaseous substrate, comprising an electrolysis cell comprising: an anode, a cathode, wherein at least one of the anode and cathode takes the form of a gas diffusion electrode, a cell space which is designed to be filled with an electrolyte and into which the anode and cathode are at least partly introduced; a feed for electrolyte designed to feed the cell space with the electrolyte; a gas space designed to feed the gas diffusion electrode with a gaseous substrate, wherein the gas space is provided on a side of the gas diffusion electrode remote from the cell space; a feed for a gaseous substrate designed to feed the gas space with the gaseous substrate; a collecting region in the gas space designed to collect electrolyte in the gas space; and a connection between the gas space and the feed for electrolyte, designed to remove electrolyte that has collected in the collecting region in the gas space from said gas space.

In some embodiments, the connection between the gas space and the feed for electrolyte comprises a Venturi nozzle.

In some embodiments, there is a closed-loop control mechanism designed to control the removal of the electrolyte from the collecting region in the gas space.

In some embodiments, the closed-loop control mechanism comprises a float which is provided in the collecting region in the gas space and is designed to periodically break the connection between the gas space and the feed for electrolyte.

In some embodiments, the float is designed as a cone or frustocone, wherein the tip of the cone or the circular face of the frustocone having the smaller size projects into the connection between the gas space and the feed for electrolyte.

In some embodiments, the closed-loop control mechanism comprises a valve in the connection between the gas space and the feed for electrolyte which is coupled to a fill level sensor in the gas space and a closed-loop controller, wherein the fill level sensor and the closed-loop controller are designed to control the valve in the connection between the gas space and the feed for electrolyte depending on the fill level of the electrolyte in the gas space.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings are intended to illustrate embodiments of the teachings of the present disclosure and impart further understanding thereof. In connection with the description, they serve to explain concepts and principles of the teachings. Other embodiments and many of the advantages mentioned are apparent with regard to the drawings. The elements of the drawings are not necessarily shown to scale relative to one another. Elements, features and components that are the same, have the same function and the same effect are each given the same reference numerals in the figures of the drawings unless stated otherwise.

FIGS. 1 to 5 show, in schematic form, illustrative diagrams of a possible construction of an electrolysis cell;

FIG. 6 shows, in schematic form, a configuration of an electrolysis plant for CO2 reduction without the configuration of the connection between electrolyte feed and gas diffusion electrode;

FIG. 7 shows, in schematic form, a configuration of an electrolysis plant for CO2 reduction with a gas diffusion electrode incorporating teachings of the present disclosure;

FIG. 8 shows, in schematic form, the construction of a Venturi nozzle incorporating teachings of the present disclosure; and

FIGS. 9 to 13 show, in schematic form, various embodiments for closed-loop control of the suction of electrolyte out of a collecting region in the gas space of an apparatus with a gas diffusion electrode and electrolyte passage.

DETAILED DESCRIPTION

In some embodiments, a process for continuous operation of an electrolysis cell with a gaseous substrate, includes an electrolyte is supplied to the electrolysis cell via an electrolyte feed, an electrolyte flow out of the electrolysis cell into the gas space takes place through a gas diffusion electrode, and the electrolyte from the electrolyte flow into the gas space is collected in a collecting region in the gas space, and the collected electrolyte is sucked out of said gas space, wherein the suction is effected via a connection between the gas space and electrolyte feed.

In some embodiments, an apparatus for continuous operation of an electrolysis cell with a gaseous substrate, comprising an electrolysis cell comprises: an anode, a cathode, wherein at least one of the anode and cathode takes the form of a gas diffusion electrode, and a cell space which is designed to be filled with an electrolyte and into which the anode and cathode are at least partly introduced; a feed for electrolyte designed to feed the cell space with the electrolyte; a gas space designed to feed the gas diffusion electrode with a gaseous substrate, wherein the gas space is provided on a side of the gas diffusion electrode remote from the cell space; a feed for a gaseous substrate designed to feed the gas space with the gaseous substrate; a collecting region in the gas space designed to collect electrolyte in the gas space; and a connection between the gas space and the feed for electrolyte, designed to remove electrolyte that has collected in the collecting region in the gas space from said gas space.

In some embodiments, a process for continuous operation of an electrolysis cell with a gaseous substrate includes an electrolyte is supplied to the electrolysis cell via an electrolyte feed, and an electrolyte flow out of the electrolysis cell into the gas space takes place through a gas diffusion electrode, the electrolyte from the electrolyte flow, especially unwanted electrolyte flow, into the gas space is collected in a collecting region in the gas space, and the collected electrolyte is sucked out of said gas space, wherein the suction is effected via a connection between the gas space and electrolyte feed. The electrolyte flow is unavoidable particularly in specific embodiments.

The processes described herein are suitable for all electrolysis cells with a gaseous substrate and especially a gas diffusion electrode, but may be used for electrolysis of CO₂ or CO, for example to CO or hydrocarbons. It is therefore described specifically in connection with CO₂ electrolysis to CO or hydrocarbons, but as stated is not restricted thereto. For carbon dioxide electrolysis, it is possible here, in some embodiments, to use gas diffusion electrodes as cathodes which comprise or consist of precious metals such as silver or gold, e.g. silver, and/or copper (for example for hydrocarbon formation in CO₂ reduction) If an oxygen-depolarized electrode is provided as gas diffusion electrode, this may consist of or at least comprise silver, for example.

Useful gaseous substrates in general include any gaseous substrates that can be employed in an electrolysis, such as carbon dioxide, carbon monoxide, oxygen, etc., e.g. carbon dioxide or carbon monoxide. The electrolyte in the process is likewise not particularly restricted and may include, for example, those that are usually used in electrolysis. In some embodiments, the electrolyte comprises an aqueous electrolyte in which conductive salts may be dissolved. Useful salts include, for example, those with alkali metal cations such as Na⁺, K⁺, etc., and with suitable anions, for example halogen anions such as Cl⁻, Br⁻, etc., sulfate and/or sulfonate ions, carbonate and/or hydrogencarbonate ions, etc., and/or mixtures thereof, and it is also possible to use ionic liquids additionally or alternatively, optionally also in solution.

A gas diffusion electrode is understood to mean an electrode through which the gaseous substrate is introduced into the electrolysis cell. In terms of its construction, the latter is not particularly restricted and especially has a porous configuration.

By a suitable adjustment of hydrophilicity and/or hydrophobicity in the gas diffusion electrode (GDE), it is also possible here, in some embodiments, to adjust the electrolyte flow from the electrolysis cell into which the electrolyte is introduced and in which the electrolysis takes place into the gas space through which the gaseous substrate is fed in. The gas diffusion electrode can thus, for example, be produced via adjustment of its hydrophobicity/hydrophilicity such that a certain electrolyte flow therethrough is enabled. The adjustment can be undertaken here in a suitable manner and is not particularly restricted.

By virtue of the passage of the electrolyte through the gas diffusion electrode, it was possible in processes known to date for salt formation or salt precipitation to occur at the gas diffusion electrode. For better understanding of the present teachings, this phenomenon is detailed hereinafter by way of example with reference to various processes in which the processes or the devices described herein can be employed.

It should be noted that the reflux of the electrolyte can simultaneously serve to eliminate or to avoid depositions of salt in the GDE. The phenomenon of salt precipitation can occur here in a wide variety of different modes of operation. For example, in the case of a chloralkali electrolysis with an oxygen-depolarized electrode, it can be described as follows:

According to prior art, in chloralkali electrolysis, the anolyte space is supplied continuously with sodium chloride as an aqueous solution. At the anode, chloride (Cl⁻) is oxidized to chlorine (Cl₂) which leaves the electrolysis cell.

2Cl⁻→Cl₂+2e ⁻ (charge neutrality)

The electrodes released are transported to the cathode through the potential source applied. In order to obtain electrical neutrality of the overall system, a membrane results in a corresponding stream of an equal number of cations.

At the cathode, water is reduced in the conventional membrane process.

H₂O+2e ⁻→H₂+2OH⁻ (charge neutrality)

The negative charges can be compensated for by cations, Na⁺ here for example, meaning that the OH⁻ ions that form at the cathode can leave the cathode space continuously, for example as sodium hydroxide solution. In the case of use of an oxygen-depolarized cathode as gas diffusion electrode, no hydrogen is formed, but rather water. The enthalpy of water formation here can lower the necessary system potential, such that less energy is consumed.

H₂O+½O₂+2e ⁻→H₂O+2OH⁻ (charge neutrality)

The oxygen-depolarized cathode in chloralkali electrolysis consists, for example, of silver, which can also be used for CO₂ reduction to CO. To balance the charge, cations, e.g. Na⁺ ions, move in the cathode space direction, and these have to be removed continuously from the electrolyte in the form of sodium hydrogencarbonate. One possible anode reaction here with oxygen, for example, is as follows:

H₂O→O₂+2H⁺+2e ⁻

Continuous operation is possible in an electrolysis, for example, when the pH is balanced by continuous mixing of catholyte and anolyte outside the electrolysis cell as well. However, this is complex. In some embodiments, however, continuous mixing of anolyte and catholyte is also possible outside the electrolysis cell. It is of course also possible for anolyte and catholyte to be identical as electrolyte, but they may also be different.

Another option is, for example, to make the anolyte acidic, such that only protons pass through a membrane in the cathode direction. It may be necessary here to introduce balancing of concentration, in order that protons, like water molecules in the case of other cations, are actively pushed through the membrane. This measure too can of course also be taken additionally in the process of the invention. Salt deposition in the gas space or gas feed space can then arise as a result of the process which follows. Hydroxide ions that form in the abovementioned examples can penetrate through to the reverse side of the porous cathode together with the corresponding cations (Na⁺, K⁺, etc.), which can form salts and be deposited on the reverse side of the electrode or else in the pores.

A further example is the electrolytic reduction of carbon dioxide. At the cathode, in all modes of operation, according to the electrode material, different products can form.

EXAMPLES

Carbon monoxide: CO₂+2e ⁻+H₂O→CO+2OH⁻

Ethylene: 2CO₂+12e ⁻+8H₂O→C₂H₄+12OH⁻

Methane: CO₂+8e ⁻+4H₂O→CH₄+4OH⁻

Ethanol: 2CO₂+12e ⁻+9H₂O→C₂H₅OH+12OH⁻

Monoethylene glycol: 2CO₂+10e ⁻+8H₂O→HOC₂H₄OH+10OH⁻

Salt deposition in the gas space can then take place by the process which follows. Hydroxide ions that form in the abovementioned examples can penetrate through to the reverse side of the porous cathode together with the corresponding cations (Na⁺, K⁺, etc.). In conjunction with CO₂, according to the pH, the corresponding hydrogencarbonate or carbonate can precipitate out (salt deposition, salting-out).

In some embodiments, the electrolyte is collected in a collecting region in the gas space and the collected electrolyte is sucked out of it in order that no electrolyte remains in the gas space, and so no salt is deposited as a result of removal of solvent. The collecting region is not particularly restricted here, provided that it can collect the electrolyte, for example as a liquid or solution, and provided that the collected electrolyte can be sucked out of it, for example through an opening or outflow with an appropriate removal device which is connected to the electrolyte feed and hence forms a connection between the gas space and the electrolyte feed. In some embodiments, the collecting region is at a lower end of the gas space, e.g. below a level (viewed from the base) of the gas diffusion electrode or below the lower end thereof, such that the electrolyte, after passing into the gas space, can flow downward by virtue of gravity, such that it does not remain for too long at the gas diffusion electrode, for example the reverse side and/or in pores thereof. The connection between the gas space and the electrolyte feed is preferably made by means of an opening or an outflow in the collecting region in the gas space, preferably at a lower end of the collecting region.

The connection between the gas space and the electrolyte feed is not particularly restricted and may be made, for example, by means of suitable pipes, hoses, etc., e.g. pipes, where the material may preferably be matched to the material of a recycled electrolyte that has been collected in the collecting region, and, for example, also correspond to the material for a feed or feed device for the electrolyte, which is likewise not particularly restricted and may take the form of a pipe, for example. In specific embodiments, the collecting region in the gas space is not in contact with the gas diffusion electrode. The pipes are not particularly restricted with regard to their further form, but in specific embodiments have a circular cross section in order to enable good transport or flow of the electrolyte.

In some embodiments, the suction is effected in that the electrolyte feed exerts a suction effect on the gas space. This can ensure that not too much electrolyte collects in the collecting region and hence comes into contact with the gas diffusion electrode again; in this way, it is also possible to minimize or prevent any influence on gas supply. In some embodiments, the gas is not supplied from the collecting region in the gas space, such that the gas does not flow or bubble through the electrolyte. Nevertheless, the collecting region is in the gas space, i.e. is also in contact with the feed for the gaseous substrate.

In some embodiments, an apparatus which can be used for the processes described herein thus comprises a collecting region for electrolyte, following gravity, at the lower end of the gas diffusion electrode or below that. The gas space and electrolyte feed may be connected in such a way that the electrolyte feed exerts a suction effect on the gas space. This can be achieved, for example, in that the connection is configured in the form of a Venturi nozzle, Laval nozzle or the like, where the connection is preferably made in the region in which the respective nozzle is narrowed and the electrolyte thus has an elevated velocity in the feed.

In some embodiments, the suction effect thus arises in that the connection between the gas space and the electrolyte feed comprises a Venturi nozzle through which the electrolyte is fed to the electrolysis cell. In this case too, the connection to the gas space may be located at a narrowed site in the Venturi nozzle. In principle, the present approach is based on the principle of the Venturi nozzle, which is shown in FIG. 8 in schematic form and by way of example including the connection to the gas space L2. The principle is based on the fact that the flow rate of a medium flowing through a pipe is inversely proportional to a varying pipe cross section. This means that the velocity is at its greatest where the cross section of the pipe is at its smallest. According to Bernoulli's law, moreover, in a flowing fluid (gas or liquid), a rise in velocity is accompanied by a drop in pressure.

Accordingly, for a nozzle according to FIG. 8, p₁>p₂ where p₁ is the pressure of the electrolyte supplied in flow direction upstream of the nozzle and p₂ is the pressure of the electrolyte in the smallest cross section of the nozzle and also the connection to the gas space L2, and v₁ is the velocity of the electrolyte in flow direction upstream of the nozzle and v₂ is the velocity of the electrolyte in the smallest cross section of the nozzle. It is possible to make use of this relationship for the suction of the electrolyte out of the collecting region in the gas space.

The Venturi nozzle, like a Laval nozzle as well, is not particularly restricted in its shape, provided that the cross section of the nozzle at first decreases in flow direction of the electrolyte supplied. The shape of the cross section is not particularly restricted and may be round, elliptical, square, rectangular, triangular, etc., but is round in specific embodiments. A symmetrical nozzle shape may be useful. In addition, the Venturi nozzle, like a Laval nozzle as well, is also not subject to any further restriction in terms of its configuration, where the connection to the gas space or collecting region in the gas space is preferably at the narrowest point of the nozzle.

The conduit L2 (where the pressure p2 exists) is connected in FIG. 8 to the gas space of the electrolysis cell in which the electrolyte collects. In some embodiments, the electrolyte is sucked out of the gas space periodically. This can ensure that the electrolyte that has passed through the GDE is sucked out regularly; on the other hand, a sufficiently long period is also available for electrolysis without interference by the suction. In specific embodiments, the suction is effected in such a way that not the entire electrolyte is sucked out of the collecting region in order to be able to better stabilize the pressure in the system and to prevent gaseous substrate from passing into the connection. In specific embodiments, passage of gaseous substrate into the connection between the gas space and the feed for electrolyte is inhibited or prevented.

In order to achieve periodic suction, the periodic suction, in some embodiments, is effected via a closed-loop control mechanism that controls the periodic suction. This closed-loop control mechanism is not subject to any further restriction and is further described in detail by way of example hereinafter with reference to various embodiments. By virtue of the closed-loop control mechanism, in some embodiments, it is especially possible to seal off or close the connection between the gas space and the feed for electrolyte from the feed for electrolyte, for example periodically, for example by means of an occlusion, for instance a liquid occlusion, that may be present at any point in the connection between the gas space and the feed for electrolyte, for example in the form of a valve, for example at a pipe connection close to the electrolyte feed, for example at a T-piece, or as an occlusion in an outflow out of the collecting region, for example, in the gas space, for example even in the form of a float.

The closed-loop control mechanism may trigger, for example, opening of the occlusion depending on measurements or sensor data, for example with reference to a fill level of the electrolyte in the collecting region, but may also be fully automatic, or else mechanical or mechanically self-regulating without any need for a measurement. Thus, the devices described herein, in some embodiments, comprise at least one closed-loop control mechanism, e.g. a liquid occlusion and a closed-loop control mechanism that opens access to the electrolyte stream as soon as a certain fill level in the collecting region has been attained. The closed-loop control mechanism here may also be integrated with the liquid occlusion, for example in the case of use of a float. In some embodiments, the closed-loop control mechanism is mechanical. This can minimize the complexity involved in the connection between gas space and feed for electrolyte.

In some embodiments, the closed-loop control mechanism comprises a float which is present in the collecting region in the gas space and enables outflow to the connection between the gas space and the electrolyte feed depending on the fill level of the electrolyte in the collecting region in the gas space, with opening of the float, for example periodically. It should be noted here that the gas space must not only adjoin the GGE but may also encompass another region of the gas feed, for example an intermediate vessel which may be provided, for example, in the base direction beneath the GDE and into which the electrolyte that has passed through can flow.

In some embodiments, the float takes the form of a cone or frustocone, for example of a stopper, where the tip of the cone or the circular face of the frustocone having the smaller size projects into the connection between the gas space and the feed for electrolyte. The frustocone can achieve a kind of “wedge shape” that closes the opening of the connection between the gas space and the feed for electrolyte in the collecting region of the gas space. In some embodiments, the float is made of a material which assures a corresponding occlusion but on the other hand is not attacked by the electrolyte and/or the gaseous substrate, for example based on elastomer or thermoset. For adjustment of the density, it is possible to use ceramic fillers. The density can also be adjusted, for example, via fluorination of the plastic. Rather than floats, it is alternatively possible to provide other closure devices such as flaps etc.

The float 9 may close off a feed of electrolyte from the collecting region 2 of the gas space 1 to the connection, especially of a Venturi nozzle or Venturi unit, in an off state, as shown in FIGS. 11 to 13, which show various embodiments with the float 9. As shown in FIG. 11, the pressure difference between the pressure in the gas chamber p_(G) and the pressure p2 in the connection to the Venturi nozzle in an off state exerts an additional force Fi on the float, caused by the pressure difference. In flow-through operation, the pressure in the gas space as a whole (not at the nozzle) is higher than in the electrolyte. With increasing amount of electrolyte in the collecting region 2, an upward force F₂ on the float 9 is generated. The float 9, or another occlusion, is designed such that the float clears the opening 3 a of the connection between the gas space and the feed of the electrolyte 3 to the Venturi nozzle over and above a certain fill level. The nozzle sucks the electrolyte out of the gas space 1 until the float 9 closes the opening again. The density of the float here is less than that of the electrolyte. By means of guides 9 a that may be made of the same material as the float, for example, it is possible to assure better occlusion of the opening 3 a.

The dimensions of the float 9 may be such that the valve opens twice per minute or less, or even once per minute or less. The utilization of a float 9 simultaneously assures hysteresis in the overall system, such that vibrations can be avoided. The float 9 and the feed of the electrolyte in the Venturi nozzle may be chosen such that the collecting region 2 is not completely emptied. This is supposed to prevent a gaseous substrate such as CO₂ from additionally being removed from the gas space 1 and the pressure p_(G) in the gas space 1 from falling significantly.

FIG. 12 shows a further embodiment of the gas space 1 with collecting region 2 in which the float 9, however, has a different stopper shape where there are adjoining cylinders on either side of the frustocone in order firstly to achieve better occlusion, but secondly also to control the buoyancy of the float by altering the mass of the float, for example. In FIG. 13, the float 9 is in conical form, which means that the electrolyte can also be recycled gradually out of the collecting region 2 to the feed of the electrolyte 3 and hence significant variations in the feed can be avoided thereby.

The apparatus with a float 9 can be utilized either in flow-through operation (with gas supply through the electrode) or in flow-by operation (with gas supply along the electrode and diffusion of the gas through the electrode). In flow-through operation, approximately the integral pressure p_(G)-p₂ in the gas space 1 is typically higher, which pushes the float 9, in addition to its own weight, against the connection to the electrolyte. It is therefore typically necessary to expend a somewhat greater force F₂ on the float 9 in order to open the opening 3 a and hence the connection. The force F₂ to be expended is also determined by the size of the opening 3 a.

In flow-by operation, the integral gas space/electrolyte pressure of ˜p_(G)-p₂ in the switched-off electrolysis system may be about the same. When the system is switched on, the potentials that occur result in electrolyte flow through the GDE back into the gas space 1. The float 9 closes the connection by virtue of the Venturi effect and by virtue of its own weight. Over and above a certain fill level, the opening 3 a is opened and electrolyte that has passed through is returned back to the electrolyte circuit.

In some embodiments, there is a further closed-loop control mechanism, such as a further occlusion, e.g. a valve, which can be closed by a closed-loop controller. It is also of course possible to provide multiple valves. In some embodiments, the suction is controlled by a valve that controls the connection between the gas space and the electrolyte feed.

In some embodiments, the valve is coupled to a fill level sensor for electrolyte in the gas space via a closed-loop controller, wherein the valve is controlled with reference to a measurement by a fill level sensor. The fill level can be measured here, for example, electronically, optically, by a pressure measurement, etc., and the fill level sensor is not particularly restricted.

A corresponding system with valves is shown in schematic form in FIGS. 9 and 10. As shown in FIG. 9, the closed-loop control unit may be accomplished by means of sensors, e.g. electrical sensors, and corresponding valves 4. The fill level in the collecting region 2 may be measured here, for example, by means of a pressure sensor 5. A correspondingly designed closed-loop controller 6, provided, for example, in the form of a p_(max)/p_(min) valve control unit, opens a valve 4 between the conduit L2 (not shown) and the gas space with collecting region 2 on exceedance of a fixed upper pressure limit p_(max). As a result of the pressure differential Δp=p_(max)−p₂, the collected electrolyte is drawn back out of the gas space 1 and, for example, fed back to the electrolyte circuit. If the pressure in the gas space 1 goes below a pressure p_(min), the valve 4 is closed again. For instance, the level of the electrolyte that passes through the GDE into the gas chamber can be kept at a predefined level and hence salt formation on the reverse side of the GDE can be prevented. In some embodiments, the fill level can alternatively be measured via a magnetic float 7 and reed switch 8, as shown by way of example in FIG. 10.

In some embodiments, the salt concentration in the electrolyte is chosen such that no salt deposition takes place during operation, i.e. during electrolysis. This concentration can be suitably determined, for example, in accordance with the solubility of a conductive salt etc. in the electrolyte.

In some embodiments, an apparatus for continuous operation of an electrolysis cell with a gaseous substrate, comprising an electrolysis cell comprises: an anode, a cathode, wherein at least one of the anode and cathode takes the form of a gas diffusion electrode, a cell space which is designed to be filled with an electrolyte and into which the anode and cathode are at least partly introduced; a feed for electrolyte designed to feed the cell space with the electrolyte; a gas space designed to feed the gas diffusion electrode with a gaseous substrate, wherein the gas space is provided on a side of the gas diffusion electrode remote from the cell space; a feed for a gaseous substrate designed to feed the gas space with the gaseous substrate; a collecting region in the gas space designed to collect electrolyte in the gas space; and a connection between the gas space and the feed for electrolyte, designed to remove electrolyte that has collected in the collecting region in the gas space from said gas space.

The apparatus described is suitable for all electrolysis cells with a gaseous substrate (CO₂, CO) and a gas diffusion electrode. In specific embodiments, the apparatus serves for CO₂ electrolysis to CO or hydrocarbons. The apparatus can be used to conduct the processes described herein. The feed for electrolyte, the gas space, the collecting region in the gas space and the connection between the gas space and the feed for electrolyte have already been discussed in connection with the process and hence correspond to those discussed above. Beyond that, these are not particularly restricted.

The feed for the gaseous substrate designed to feed the gas space with the gaseous substrate is not particularly restricted either, provided that it is capable of supplying gas and is preferably not impaired by the gas, and may be designed, for example, as a pipe, hose or the like. Furthermore, the apparatus may also have a removal device for electrolyte and/or a liquid or dissolved product and/or a removal device for a gaseous product and/or unconsumed gaseous substrate, which are not particularly restricted.

In some embodiments, the electrolysis cell comprises at least an anode and a cathode, at least one of which takes the form of a gas diffusion electrode, and a cell space designed to be filled with an electrolyte and into which the anode and the cathode have been at least partly introduced. In some embodiments, both the anode and the cathode take the form of a gas diffusion electrode. In some embodiments, the anode takes the form of a gas diffusion electrode. In some embodiments, the cathode takes the form of a gas diffusion electrode. In some embodiments, carbon dioxide or carbon monoxide is electrolytically converted at the cathode, i.e. the cathode is designed such that it can convert carbon dioxide, for example in the form of a copper-containing (CO₂, CO) and/or silver-containing (CO₂) gas diffusion electrode.

The electrolysis cells used correspond, for example, to those of the prior art that are shown in schematic form in FIGS. 1 to 5; the figures show cells with a membrane M, which may also be absent in the apparatus of the invention, but is employed in specific embodiments, and which can separate an anode space I and a cathode space II. If such a membrane is present, this is not particularly restricted and is matched, for example, to the electrolysis, for example the electrolyte and/or the anode reaction and/or cathode reaction.

The electrochemical reduction, for example of CO₂, takes place in an electrolysis cell which typically consists of an anode and a cathode space. FIGS. 1 to 5 show examples of a possible cell arrangement. For each of these cell arrangements, it is possible to use a gas diffusion electrode described herein, for example as cathode.

By way of example, the cathode space II in FIGS. 1 and 2 is configured such that a catholyte is supplied from below and then leaves the cathode space II in the upward direction. In some embodiments, the catholyte can be supplied from above, as in the case of falling-film electrodes for example. At the anode A, which is electrically connected to the cathode K by means of a power source for provision of the potential for the electrolysis, the oxidation of a substance which is supplied from below, for example with an anolyte, takes place in the anode space I, and the anolyte with the product of the oxidation then leaves the anode space. In the 3-chamber construction shown in FIGS. 1 and 2, a reaction gas, for example carbon dioxide, can be conveyed into the cathode space II for reduction through the gas diffusion electrode, here by way of example the cathode K (not shown in detail as the GDE), by way of example in flow-by operation as in FIG. 1 or in flow-through operation in FIG. 2. In some embodiments, there is one or more porous anodes A.

In FIGS. 1 and 2, the spaces I and II are separated by a membrane M. By contrast, in the PEM (proton or ion exchange membrane) construction of FIG. 3, the gas diffusion electrode as cathode K (likewise not shown in detail as the GDE) and a porous anode A directly adjoin the membrane M, which results in separation of the anode space I from the cathode space II. The construction in FIG. 4 corresponds to a mixed form of the construction from FIG. 2 and the construction from FIG. 3, with provision of a construction with the gas diffusion electrode and gas feed G in flow-through operation on the catholyte side, as shown in FIG. 2, whereas a construction as in FIG. 3 is provided on the anolyte side.

In some embodiments, mixed forms or other configurations of the electrode spaces described by way of example are also conceivable. Embodiments without a membrane are also conceivable. In some embodiments, the electrolyte on the cathode side and the electrolyte on the anode side may thus be identical, and the electrolysis cell/electrolysis unit may not need a membrane. However, it is not ruled out that the electrolysis cell in such embodiments has a membrane, although this is associated with additional expenditure with regard to the membrane and also the potential applied. Catholyte and anolyte may also optionally be mixed again outside the electrolysis cell.

FIG. 5 corresponds to the construction of FIG. 4, except that, again, as in FIG. 1, the gas supply G here takes place in flow-by operation and the passage of reactants and products E and P is shown.

FIGS. 1 to 5 are schematic diagrams. The electrolysis cells from FIGS. 1 to 5 may also be combined to form mixed variants. For example, the anode space may be designed as a PEM half-cell, as in FIG. 3, while the cathode space consists of a half-cell that includes a certain electrolyte volume between membrane and electrode, as shown in FIG. 1. The membrane may also have a multilayer design, such that separate feeds of anolyte or catholyte are enabled. Separation effects are achieved in aqueous electrolytes, for example, via the hydrophobicity of interlayers. Conductivity can nevertheless be assured when conductive groups are integrated into such separation layers. The membrane may be an ion-conducting membrane, or a separator that brings about merely mechanical separation and is permeable to cations and anions.

The use of the gas diffusion electrode described herein makes it possible to construct a three-phase electrode. For example, a gas can be guided from the rear to the electrically active front side of the electrode in order to conduct an electrochemical reaction there. In some embodiments, the gas may also merely flow by the gas diffusion electrode, meaning that a gas such as CO₂ is guided past the rear of the gas diffusion electrode in relation to the electrolyte, in which case the gas can penetrate through the pores of the gas diffusion electrode and the product can be removed at the back. In some embodiments, the gas flow in the flow-by regime is in the reverse direction to the flow of the electrolyte, in order that a liquid that has been forced through, such as electrolyte, can be transported away.

In specific embodiments, the connection between the gas space and the feed for electrolyte comprises a Venturi nozzle or another nozzle, for instance a Laval nozzle, preferably a Venturi nozzle.

In some embodiments, the apparatus may further comprise a closed-loop control mechanism designed to control the removal of the electrolyte from the collecting region in the gas space. This closed-loop control mechanism is not subject to any further restriction and corresponds, for example, to the descriptions given in connection with the processes described herein. By virtue of the closed-loop control mechanism, it is especially possible, in some embodiments, to seal off or close the connection between the gas space and the feed for electrolyte from the feed for electrolyte, for example periodically, for example by means of an occlusion, for instance a liquid occlusion, that may be present at any point in the connection between the gas space and the feed for electrolyte, for example in the form of a valve, for example at a pipe connection close to the electrolyte feed, for example at a T-piece, or as an occlusion in an outflow out of the collecting region, for example, in the gas space, for example even in the form of a float. The closed-loop control mechanism may trigger, for example, opening of the occlusion depending on measurements or sensor data, for example with reference to a fill level of the electrolyte in the collecting region, but may also be fully automatic, or else mechanical without any need for a measurement.

Thus, in some embodiments, there is at least one closed-loop control mechanism, e.g. a liquid occlusion and a closed-loop control mechanism that opens access to the electrolyte stream as soon as a certain fill level in the collecting region has been attained. The closed-loop control mechanism here may also be integrated with the liquid occlusion, for example in the case of use of a float. In some embodiments, the closed-loop control mechanism comprises a float which is provided in the collecting region in the gas space and is designed to periodically stop the connection between the gas space and the feed for electrolyte. The float here may be of any desired design provided that it can stop the connection between the gas space and the feed for electrolyte. In some embodiments, the float takes the form of a cone or frustocone, where the tip of the cone or the circular face of the frustocone having the smaller size projects into the connection between the gas space and the feed for electrolyte.

In some embodiments, the closed-loop control mechanism comprises a valve in the connection between the gas space and the feed for electrolyte, coupled to a fill level sensor in the gas space and a closed-loop controller, wherein the fill level sensor and the closed-loop controller are designed to control the valve in the connection between the gas space and the feed for electrolyte depending on the fill level of the electrolyte in the gas space.

In some embodiments, the apparatus comprises multiple electrolysis cells or a stack of electrolysis cells, in each of which at least one of the anode and cathode takes the form of a gas diffusion electrode, in which case each of these electrolysis cells has at least one gas space connected in each case either to one feed for electrolyte for all cells or to multiple feeds for electrolyte for all cells, for example including separate feeds for electrolyte for each cell, via a connection between the gas space and the corresponding feed for electrolyte. The multiple electrolysis cells may then be combined to form a cell stack (e.g. 100 or more cells) in order to save space. In the case of such a stack, especially for reasons of space, the use of floats as control mechanism or self-regulating system is advantageous. In specific embodiments, it is thus also possible to employ apparatuses having multiple electrolysis cells or cell stacks having, for example, 100 or more cells, where the use of floats is likewise advantageous here for the control of the connection between the gas space and the corresponding feed for electrolyte.

In some embodiments, the apparatus may comprise further constituents present in an electrolysis plant, i.e. not only the power source for the electrolysis but also various cooling and/or heating devices etc. These further constituents of the apparatus, for example of an electrolysis plant, are not subject to any further restriction and may be provided in a suitable manner.

The above embodiments, configurations, and developments can, if viable, be combined with one another as desired. Further possible configurations, developments, and implementations of the teachings herein also include combinations that have not been mentioned explicitly or features that have been described above or are described hereinafter for the working examples. More particularly, the person skilled in the art will also add individual aspects as improvements or supplementations to the respective basic form of the present teachings. The description hereinafter makes reference to some illustrative embodiments, but these do not restrict the scope of the teachings.

EXAMPLES

All experiments, and also the comparative examples and examples, were conducted at room temperature of about 20° C.-25° C., unless stated otherwise.

Comparative Example 1

An apparatus for CO₂ electrolysis without connection of electrolyte feed and gas feed is shown in FIG. 6.

An electrolysis is shown, in which carbon dioxide is reduced on the cathode side and water is oxidized on the anode side A. On the anode side, it would be possible, for example, for a reaction of chloride to give chlorine, bromide to give bromine, sulfate to give peroxodisulfate (with or without evolution of gas), etc. to take place. Platinum is an example of a suitable anode A, and copper of a suitable cathode K. The two electrode spaces of the electrolysis cell are separated by a membrane M made of Nafion®. The incorporation of the cell into a system with an anolyte circuit 10 and catholyte circuit 20 is shown in FIG. 6 without a gas diffusion electrode (or FIG. 7 with the gas diffusion electrode; see comparative example 2).

On the anode side, water with electrolyte additions is fed into an electrolyte reservoir vessel 12 via an inlet 11. However, it is not ruled out that water is supplied additionally or instead of the inlet 11 at another point in the anolyte circuit 10, since, according to FIG. 6, the electrolyte reservoir vessel 12 can also be used for gas separation. The electrolyte is pumped out of the electrolyte reservoir vessel 12 by means of the pump 13 into the anode space, where it is oxidized. The product is then pumped back into the electrolyte reservoir vessel 12, where it can be removed to the product gas vessel 26. Via a product gas outlet 27, the product gas can be withdrawn from the product gas vessel 26. The product gas can of course also be removed elsewhere. The result is thus an anolyte circuit 10 since the electrolyte is circulated on the anode side.

On the cathode side, in the catholyte circuit 20, carbon dioxide is introduced via a CO₂ inlet 22 into an electrolyte reservoir vessel 21, where it is physically dissolved, for example. By means of a pump 23, this solution is brought into the cathode space, where the carbon dioxide is reduced at the cathode K, for example to CO at a silver cathode. An optional further pump 24 then pumps the solution containing CO and unconverted CO₂ obtained at the cathode K further to a vessel for gas separation 25, where the product gas comprising CO can be removed into a product gas vessel 26. Via a product gas outlet 27, the product gas can be removed from the product gas vessel 26. The electrolyte is in turn pumped from the vessel for gas separation back to the electrolyte reservoir vessel 21, where carbon dioxide can be added again. Here too, merely an illustrative arrangement of a catholyte circuit 20 is specified, where the individual apparatus components of the catholyte circuit 20 may also be arranged differently, for example in that the gas separation is already effected in the cathode space.

In some embodiments, the gas separation and the gas saturation are effected separately; in other words, in one of the vessels, the electrolyte is saturated with CO₂ and then pumped through the cathode space as a solution without gas bubbles. The gas that exits from the cathode space then consists predominantly of CO since CO₂ itself remains dissolved since it has been consumed and hence the concentration in the electrolyte is somewhat lower. The electrolysis is effected in FIG. 6 by addition of current via a power source which is not shown.

In order to feed the electrolyte and the CO₂ dissolved in the electrolyte to the electrolysis unit with variable pressure over time, valves 30 are included in the anolyte circuit 10 and catholyte circuit 20, which are controlled by a control device which is not shown and hence control the feed of anolyte and catholyte to the anode and cathode respectively, as a result of which the feed is effected with variable pressure and product gas can be purged out of the respective electrode cells.

The valves 30 are shown in the figure upstream of the inlet into the electrolysis cell, but may also be provided, for example, downstream of the outlet from the electrolysis cell and/or at other points in the anolyte circuit 10 or catholyte circuit 20. It is also possible, for example, for a valve 30 to be present in the anolyte circuit upstream of the inlet into the electrolysis cell, while the valve in the catholyte circuit 20 is beyond the electrolysis cell, or vice versa.

In the operation of the cell, there was salt formation at the cathode K.

Comparative Example 2

The apparatus shown in FIG. 7 corresponds to the apparatus in comparative example 1, where the cathode K here takes the form of a flow-through gas diffusion electrode. In the operation of the cell, there was likewise salt formation at the cathode K.

Example 1

The construction in example 1 corresponds to that in comparative example 2, except that the valves 30 are dispensed with and a Venturi nozzle is provided in the catholyte circuit 20 in place thereof, which is connected to the feed for CO₂ in the gas space in accordance with the construction in FIG. 11. Under “normal” operating conditions, operability over 1000 h was demonstrated.

In the operation of customary electrolysis cells, it has been found that electrolyte passes through gas diffusion electrodes (GDE) and leads to electrolyte collection in the gas space. Both in the case of flow-through operation and flow-by operation, the electrolyte loses solvent, especially water, as a result of the gas flow, as a result of which depositions of salt occur in the gas space and particularly disadvantageously in the GDE itself. These lead to loss of selectivity and ultimately to failure of the electrode or electrolyzer.

This problem can be solved by supplementing the electrolysis cell with a stackable, e.g. purely mechanical connection between gas space and electrolyte feed, which can suck out liquid that has passed through into the gas space, preferably periodically. The apparatus may comprise a suction unit that works by the “Venturi principle” and a float that ensures the necessary hysteresis. The amount of electrolyte that flows through the GDE can be achieved, for example, via an adjustment of the hydrophilicity of the GDE. 

What is claimed is:
 1. A method for continuous operation of an electrolysis cell with a gaseous substrate, the method comprising: supplying an electrolyte to the electrolysis cell via an electrolyte feed; flowing the electrolyte out of the electrolysis cell into the gas space through a gas diffusion electrode; collecting the electrolyte from the electrolyte flow into the gas space in a collecting region in the gas space; and sucking the collected electrolyte out of said gas space via a connection between the gas space and electrolyte feed.
 2. The method as claimed in claim 1, wherein the electrolyte feed exerts a suction effect force on the gas space.
 3. The process as claimed in claim 2, wherein the connection between the gas space and the electrolyte feed comprises a Venturi nozzle through which the electrolyte is fed to the electrolysis cell.
 4. The process as claimed in claim 1, wherein the electrolyte is sucked out of the gas space periodically.
 5. The process as claimed in claim 4, wherein the periodic suction is triggered via a closed-loop control mechanism.
 6. The process as claimed in claim 5, wherein the closed-loop control mechanism is mechanical.
 7. The process as claimed in claim 6, wherein: the closed-loop control mechanism comprises a float in the collecting region in the gas space; and the control mechanism enables outflow to the connection between the gas space and the electrolyte feed depending on the fill level of the electrolyte in the collecting region in the gas space.
 8. The process as claimed in claim 1, wherein the suction is actuated by a valve controlling the connection between the gas space and the electrolyte feed.
 9. The process as claimed in claim 8, wherein: the valve is coupled to a fill level sensor for electrolyte in the gas space via a closed-loop controller; and the valve is controlled with reference to a measurement by a fill level sensor.
 10. An apparatus for continuous operation of an electrolysis cell with a gaseous substrate, the apparatus comprising: an anode and a cathode, wherein at least one of the anode and cathode takes the form of a gas diffusion electrode; a cell space configured be filled with an electrolyte and into which the anode and cathode are at least partly disposed; an electrolyte feed into the cell space; a gas space to feed the gas diffusion electrode with a gaseous substrate, the gas space disposed on a side of the gas diffusion electrode remote from the cell space; a feed to feed the gas space with the gaseous substrate; a collecting region in the gas space configured to collect electrolyte in the gas space; and a connection between the gas space and the electrolyte feed to remove electrolyte that has collected in the collecting region in the gas space from said gas space.
 11. The apparatus as claimed in claim 10, wherein the connection between the gas space and the feed for electrolyte comprises a Venturi nozzle.
 12. The apparatus as claimed in claim 10, further comprising a closed-loop control mechanism to control the removal of the electrolyte from the collecting region in the gas space.
 13. The apparatus as claimed in claim 12, wherein: the closed-loop control mechanism comprises a float disposed in the collecting region in the gas space; and the control mechanism periodically breaks the connection between the gas space and the feed for electrolyte.
 14. The apparatus as claimed in claim 13, wherein the float comprises a cone or frustocone, wherein the tip of the cone or the circular face of the frustocone having the smaller size projects into the connection between the gas space and the feed for electrolyte.
 15. The apparatus as claimed in claim 12, wherein: the closed-loop control mechanism comprises a valve in the connection between the gas space and the feed for electrolyte coupled to a fill level sensor in the gas space and a closed-loop controller; the fill level sensor and the closed-loop controller control the valve in the connection between the gas space and the feed for electrolyte depending on the fill level of the electrolyte in the gas space. 